The present invention concerns a method for internal fixation of vertebra of the spine.
It has long been known that internal fixation is an adjunct to fusion, such as a transverse process fusion. In early prior art techniques, a surgeon made an incision in the patient""s back and separated tissue and muscle in order to expose a wide area of the spine in which the procedure was to take place. The fusion and fixation in one prior art process is by grafting bone segments between opposing transverse processes of adjacent vertebrae. However, this technique resulted in gross destruction of normal anatomy as well as high blood loss. Moreover, this surgical technique did not completely stabilize the vertebra since there was no direct connection between the vertebral bodies.
In more recent times, a surgical technique known as dowel inter body fusion has been developed. In this technique, bores are formed in disc tissue through either open surgery or percutaneous surgery. A dowel is made to fit into the bores formed in the disc tissue. In still a further technique, all disc tissue is removed between adjacent vertebrae, as well as the disc plates. Large surface area bone grafts are then placed within the clean space to form a graft between the opposing vertebral bodies. In each of these latter two prior art processes it still remains necessary to provide some means for fixation to facilitate fusion of the large area bone graft or the dowel to the vertebrae.
Many types of instrumentation for performing spinal fixation are known in the art. For instance, spine instrumentation developed by Harrington incorporates a hook and rod configuration. Implantation of the Harrington spinal instrumentation requires subperiosteal stripping of the spine to avoid injury to the muscular nerves and vessels. Dissection of the muscle tissue is also required. In some aspects of the early Harrington techniques, the spine was stripped clean of the supraspinous and intraspinous ligaments.
Later developed techniques involve hardware which is placed through the skin and through the muscle into the bone. Some of the fixation hardware remains outside the body, but is removed after the fusion has been completed. Techniques of this sort are characterized by high risk of pin tract infection and incisional morbidity.
Thus far, each of the prior art spinal fixation and or fusion techniques have been characterized by excessive invasion into the patients spine and back region. What is needed is a technique which allows for adequate stabilization of the spine, yet decreases the chance of infection as well as patient morbidity. There is further a need for such a method which permits percutaneous removal of the fixation hardware as an outpatient procedure after fusion has been completed.
The present invention contemplates a percutaneous fusion technique using subcutaneous suprafascial internal fixation. More particularly, the minimally invasive technique of the present invention permits anterior fusion of the disc space of the lumbar spine following appropriate disc resection and bone grafting. The fixation process is suprafascial, that is above the muscle fascia, but subcutaneous, that is beneath the surface of the skin. Thus, none of the muscle tissue is destroyed and the subcutaneous nature of the procedure greatly decreases the risk of pin tract secretions or infections, or the potential of osteomyelitis.
In more specific aspects of the invention, the technique contemplates first resecting the intranuclear cavity of a damaged disc, including ablation of the superior and inferior end plates. Bone graft material is prepared for introduction into the vacated disc space. Prior to introduction of the bone graft into the empty disc nuclear space, fixation instrumentation is implanted. In general, this fixation hardware can include self-tapping cannulated bone screws, fixation plates and linking members for laterally fixing plates on opposite sides of the spinous process.
In the preferred method, guide wires are inserted bilaterally in line with and into the pedicle. Pedicle screws are advanced over the guide wire and engaged into a predrilled bore in the pedicle. After the guide wire is removed, the skin is elevated and tissue in the suprafascial subcutaneous space is dissected to permit insertion of the fixation plates. The appropriate plates are first engaged over the ipsilateral screws and then the procedure is repeated for the contralateral bone screws at each level of hardware, that is at each vertebra to be stabilized. The bilateral fixation plates can be laterally connected by dissecting across the midline between corresponding screws and then positioning a linking member between the screws using a top-loading insertion mechanism. A nut is also top-loaded on to each successive screw to secure the Linking members to the plate and to secure the plate to the pedicle screws.
In the inventive method, the nuts engaging the pedicle screws are initially loosely threaded onto the screws. The bone screws are then advanced into the vertebral body until the hardware resides below the level of the skin, but suprafascially in the subcutaneous space at each level of the instrumentation. The nuts are then tightened when they fixation hardware is in its final resting spot. Once the fixation instrumentation is in position, the bone graft material is introduced through a cannula to the disc space and moved into position by an obturator. With the bone graft in place and the spinal fixation hardware engaged to the appropriate vertebrae, the subcutaneous tissue is then irrigated and closed.
In another aspect of the invention, a cannulated fixation or bone screw is provided which is well suited for use with the inventive method. More specifically, the screw includes a distal threaded shank and a proximal nut threaded stem which terminates in a driving hub. The distal threaded shank includes self-tapping bone engaging threads. Intermediate the threaded shank and the stem is a smooth shank of sufficient length so that only the smooth shank contacts muscle tissue when the fixation instrumentation is in place. Near the stem end of the smooth shank is a mounting hub which supports the fixation plate before the nut is engaged on the threaded stem. Tile smooth shank preferably accounts for about one-half of the length of the screw as measured from the tip of the bone engaging threaded shank to the underside of the mounting hub.
In a further aspect of the invention, a three component dilator system is provided to facilitate instrumentation of the vertebrae. In particular, the dilator system includes three concentrically disposed hollow dilator tubes, each tapered at its respective end for atraumatic introduction into the patient. Each of the three dilators is successively smaller in diameter but larger in length. The intermediate and smallest dilator tubes have knurled ends to grasp for removal during steps of the method.
It is one object of the present invention to provide a method for internal fixation of the spinal column which is minimally invasive and which poses a minimal health risk to the patient. Another object is to provide such a technique which further permits subcutaneous removal of the temporarily implanted hardware in an out-patient procedure.
A further object of the invention is realized by the present technique which contemplates subcutaneous but suprafascial fixation to avoid damage to the spinal musculature and ligaments. Further objects and certain advantages of the present invention will become apparent from the following description of the invention.